Multilayer plastic films have been previously disclosed. For example, Japanese Patent KOKAI numbers JA56-118476 and JA56-27326 disclose oriented multilayer tape constructions containing five or less layers in the film wherein dissimilar thermoplastic materials are laminated together to form a film, the film biaxially oriented and subsequently used to make a pressure sensitive adhesive tape.
These films contain five or fewer layers due to limitations in the laminating techniques employed, e.g., film lamination, hot roll film pressing and the like. Moreover, the laminate must be oriented before being used.
Multilayer films comprising more than 5 layers are also known. See, for example, U.S. Pat. Nos. 3,480,502, 3,487,505, 3,565,985, 3,576,707, 3,647,612, 3,759,647 and 3,801,429. An important aspect of these disclosures is that iridescent multilayer articles can be produced by techniques in which two or more thermoplastic resins are joined in a multiplicity of thin layers. Some of these references also disclose that an adhesive may be used to attach these articles to a substrate. However, these references describe neither severable films nor the properties which both the film and its various components must possess in order.. to be dispensable. To the contrary, an article by one of the named inventors in these patents teaches that the combination of materials used in the present invention would not provide the film of this invention. See W. J. Schrenk and T. Alfrey, Jr., Polymer Engineering and Science, November, 9, p. 397, 1969.
Multilayer films are also discussed in an article by Eric Baer, Scientific American, October, 1986, p. 179 et. seq. at 183 and 186. This article discusses a sheet of alternating layers of two different polymers (one brittle and one ductile) and states that the ductile layer absorbs energy and stops crack propagation. It further states that if one component is stiff and the other rubbery the bulk material may be both stiff and tough. In both cases, the article teaches that the combination of materials used in the present invention would not provide the film of the invention.
Although multilayer films have been disclosed as discussed above, a need still remains for a severable multilayer film. For example, a severable film is desirable whenever one wishes to sever a section or length of a particular article from a wound roll. A severable film is, therefore, useful as a backing for pressure-sensitive adhesive tapes such as are used in homes and offices. Currently the films employed in such tapes are expensive, relatively difficult to dispense, or both. Additionally, many of the films used in such tapes (e.g., polypropylene, polyvinyl chloride, etc.) become substantially more difficult to dispense when the dispensing blade used becomes dull or damaged.
This dispensing difficulty manifests itself in several ways. For example, a significantly greater force may be necessary in order to bring about dispensing. Additionally, the tape or film may sever irregularly. That is, it may chip, fracture or break in an unpredictable manner rather than severing cleanly and in a straight line. The tape or film may also stretch, string out, or stress whiten at the line of severance. These difficulties are undesirable both from a functional and an aesthetic viewpoint.
A severable film is, of course, useful in a variety of other ways. It may be used as a decorative ribbon or sheet. It may be used when it is desired to remove a portion of a film or sheet from the central portion of a larger sheet. Severable films are not, however, limited to such applications. Rather they have utility wherever a severable film is desired.